1. Field of the Invention
The present invention relates to an oscillator device utilizing a crystal oscillator, more specifically, to a crystal oscillator device, which can simultaneously generate a plurality of signals with different oscillation modes of the crystal oscillator.
2. Description of the Related Art
The crystal oscillator used in oscillator devices generates not only main oscillation but also sub-oscillation. Generally, only the main oscillation of oscillator devices is utilized, and therefore it is required that the sub-oscillation does not cause deterioration of the main oscillation, this is achieved by controlling the sub-oscillation. However, extraction and utilization of the signal of such a sub-oscillation for the purpose of temperature compensation and so forth have been proposed.
It is widely known that the crystal oscillators such as SC-cut and IT-cut crystal oscillators, for example, generate sub-oscillation (B-mode) with a frequency of 9% in higher frequency of resonance frequency of the oscillation mode (C-mode) of the main oscillation. It is proposed in Patent Document 1 (U.S. Pat. No. 4,079,280) that as the frequency of the B-mode signal as a function of temperature is linear, the oscillation frequency of the C-mode can be stabilized by temperature-compensation based on the nature of the functional relationship between the frequency of the B-mode signal and the temperature.
FIG. 1 is a circuit block diagram of the crystal oscillator described in Patent Document 1, etc.
In the oscillator device described in FIG. 1, a feedback signal is provided to an SC-cut crystal oscillator 121 after amplification by amplifier 123. The output from the crystal oscillator 121 is filtered utilizing two filters 122a and 122b, and is output as B-mode and C-mode signals respectively.
The oscillator device in Patent Document 1 can perform more accurate temperature compensation, compared with a method, which estimate the temperature using a temperature sensor equipped by the crystal oscillator, because it can detect the actual temperature of a resonator of crystal consisting the crystal oscillator from the frequency of the B-mode signal with high accuracy.
The oscillator devices, utilizing an AT-cut crystal, in Patent Document 2 (U.S. Pat. No. 4,525,647) and Patent Document 3 (U.S. Pat. No. 4,872,765) simultaneously generate two signals, output from the crystal oscillator device, by two different oscillation modes of an AT-cut crystal, namely thickness shear and face contour oscillation modes.
The method of simultaneously generating a plurality of signals with different oscillation modes of an oscillator is described in the above Patent Documents 2 and 3. However, none of the Patent Documents 1 to 3 describe the detailed configuration of a circuit to realize the simultaneous generation of two modes. For example, in the configuration in Patent Document 1, the signals of the B mode and the C-mode are output separately, as shown in FIG. 1. However, because they are common outputs, the output and the B-mode and C-mode feedback signals of FIG. 1 are in practice the same signal, that is a signal with B-mode and C-mode combined. In order to obtain individual mode signal outputs, the signals should be separated by performing processing on the shared output. In Patent Document 1, however, does not describe any method for so doing.
In Patent Document 2, the configuration of the crystal oscillator comprises, independent terminals for thickness shear oscillations and for contour oscillations, and four input and output terminals. However, in reality such a crystal oscillator does not exist.
When the simultaneous generation of signals with two different oscillation modes from a crystal oscillator is attempted, a see-saw phenomenon in which either one or the other of the signals is generated because of fluctuations in supply voltage and surrounding temperature. The range of supply voltage and surrounding temperature within which the signals of two modes are simultaneously generated is very narrow, and if the temperature or the voltage fluctuate beyond this range, either one of the signals or neither of the signals is generated. Especially, in order to unfailingly generate upon power supply, very difficult fine control is required However, none of the Patent Documents above has any description of a means for addressing the see-saw phenomenon. Therefore with the descriptions of the above Patent Documents, it is difficult to guarantee the stable generation of the signals of two oscillation modes without failure and to maintain the condition as stable as possible.
The temperature compensation of the crystal oscillator, for stabilizing the oscillation frequency of the crystal oscillator, comprises a heater in the proximity of the crystal oscillator, thermally connecting the heater with the crystal oscillator and an oscillation circuit, and maintains a certain high temperature of the heater.
The types of heater used are the rim type, where the heater surrounds the crystal oscillator, and the heat tube type, where the oscillator circuit is sealed inside a metal package and a heater wire is coiled around the package. However, such heaters have a large thermal resistance in the part between the heater and a piece of crystal inside the crystal oscillator package, which is kept at high vacuum, thus thermal efficiency is low and lack long-term stability. Further the complicated assembly increases cost.